Fused quartz article having controlled devitrification

ABSTRACT

A fused quartz article, such as a muffle tube or crucible, with enhanced creep resistance. The enhanced creep resistance is the result of controlled devitrification of the fused quartz article. Controlled devitrification is achieved by coating the article with a colloidal silica slurry doped with metal cations, such as barium, strontium, and calcium. The metal cations in the slurry promote nucleation and growth of cristobalite crystals into the fused quartz at temperatures in the range from about 1000° C. to about 1600° C. The cristobalite has significantly. higher viscosity, and therefore greater creep resistance at elevated temperatures, than fused quartz. Methods for applying a doped coating to a fused quartz article and improving the creep resistance of a fused quartz article are also disclosed.

BACKGROUND OF INVENTION

The invention relates to an article formed from fused quartz. Moreparticularly, the invention relates to an article comprising fusedquartz and having an outer coating containing metal cations. Even moreparticularly, the invention relates to a fused quartz article having anouter coating containing metal cations, wherein the fused quartzundergoes a transition to a cristobalite crystal structure at atemperature of at least about 1000° C. The invention also relates to amethod of coating a fused quartz article with a coating containing aplurality of metal cations.

Quartz (also referred to hereinafter as “fused quartz”) is used to formarticles, such as tubes and crucibles, which are used in the processingof products, such as optical fibers and semiconducting materials, athigh temperatures. Quartz articles undergo viscous creep at hightemperatures and are thus prone to failure. Quartz muffle tubes, forexample, are used throughout the fiber optics industry during thesintering step in optical-quality glass boule production. In thesintering application, tubes are usually suspended vertically, andsintering takes place at temperatures around 1500° C. At suchtemperatures, the fused quartz muffle tubes undergo viscous creep, whichcan lead to muffle tube failure.

Fused quartz articles, such as muffle tubes and crucibles, have limitedservice lifetimes due to viscous creep occurring at high temperatures ofoperation. Therefore, what is needed is a fused quartz article that isresistant to creep at high temperatures. What is also needed is a methodof treating a fused quartz article to improve the creep resistance ofthe article.

SUMMARY OF INVENTION

The present invention meets these and other needs by providing a fusedquartz article, such as a muffle tube or crucible, with enhanced creepresistance. The enhanced creep resistance is the result of controlleddevitrification of the fused quartz article. Controlled devitrificationis achieved by coating the tubes with a colloidal silica slurry dopedwith metal cations, such as, but not limited to, barium, strontium, andcalcium. The metal cations in the slurry promote nucleation and growthof cristobalite crystals (also referred hereinafter as “cristobalite”)into the fused quartz at elevated temperatures. The cristobalite hassignificantly higher viscosity, and therefore greater creep resistanceat elevated temperatures, than fused quartz.

Accordingly, one aspect of the invention is to provide a fused quartzarticle. The fused quartz article comprises: a body, the body comprisingfused quartz; and a coating disposed on an exposed surface of the body,the coating comprising a plurality of metal cations, each having avalence of less than 4, wherein the plurality of metal cations comprisescations of at least one of an alkali metal, an alkaline earth metal, arare earth metal, and combinations thereof. The plurality of metalcations is present within the coating in a concentration of at leastabout 0.1 atomic percent. The fused quartz within the body undergoes atransition to a cristobalite crystal structure at a temperature of atleast about 1000° C.

A second aspect of the invention is to provide an outer coating for afused quartz article. The outer coating comprises a plurality of metalcations, wherein the plurality of metal cations comprises cations of atleast one of barium, calcium, strontium, and combinations thereof. Theplurality of metal cations is present within the coating in aconcentration of at least about 0.1 atomic percent. The plurality ofcations catalyzes a transition of fused quartz within the fused quartzarticle to a cristobalite crystal structure at a temperature of at leastabout 1000° C.

A third aspect of the invention is to provide a fused quartz article.The fused quartz article comprises: a body comprising fused quartz; andan outer coating disposed on an exposed surface of the body, the outercoating comprising a plurality of metal cations, wherein the pluralityof metal cations comprises cations of at least one of barium, calcium,strontium, and combinations thereof. The plurality of metal cations ispresent within the coating in a concentration of at least about 0.1atomic percent. The plurality of cations catalyzes a transition of fusedquartz within the body to a cristobalite crystal structure at atemperature of at least about 1000° C. The fused quartz article istransparent to visible light.

A fourth aspect of the invention is to provide a method of forming adoped coating on an exposed surface of a fused quartz article having abody comprising fused quartz and a doped coating disposed on an exposedsurface of the body. The doped coating comprises a plurality of metalcations, each having a valence of less than 4, wherein the plurality ofmetal cations comprises cations of at least one of an alkali metal, analkaline earth metal, a rare earth metal, and combinations thereof, andwherein the plurality of metal cations is present within the coating ina concentration of at least about 0.1 atomic percent. The methodcomprises the steps of: providing a silica slurry, the silica slurrybeing doped with a plurality of metal cations comprising cations of atleast one of an alkali metal, an alkaline earth metal, a rare earthmetal, and combinations thereof; providing a fused quartz article;applying the silica slurry to an exposed surface of the fused quartzarticle; drying the silica slurry on the exposed surface; and firepolishing the exposed surface to form the doped coating on the exposedsurface.

A fifth aspect of the invention is to provide a method of improving thecreep-resistance of a fused quartz article. The fused quartz articlecomprises a body of fused quartz and a coating disposed on an exposedsurface of the body, wherein the coating comprises a plurality of metalcations, each having a valence of less than 4, wherein the plurality ofmetal cations comprises cations of at least one of an alkali metal, analkaline earth metal, a rare earth metal, and combinations thereof, andwherein the plurality of metal cations is present within the coating ina concentration of at least about 0.1 atomic percent. The methodcomprises the steps of: providing a silica slurry, the silica slurrybeing doped with a plurality of metal cations comprising cations of atleast one of an alkali metal, an alkaline earth metal, a rare earthmetal, and combinations thereof; providing a fused quartz article;applying the silica slurry to an exposed surface of the fused quartzarticle; drying the silica slurry on the exposed surface; fire polishingthe exposed surface, wherein the silica slurry, after drying, forms acoating on the outer surface; and nucleating cristobalite crystals at atemperature of at least about 1000° C. on the outer surface, wherein thecristobalite crystals enhance the creep resistance of the fused quartzarticle.

These and other aspects, advantages, and salient features of the presentinvention will become apparent from the following detailed description,the accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a micrograph, taken at 500× magnification, of a cross-sectionof a fused quartz muffle tube of the present invention.

DETAILED DESCRIPTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures. It is also understood that terms such as “top,” “bottom,”“outward,” “inward,” and the like are words of convenience and are notto be construed as limiting terms.

The present invention provides a fused quartz article having enhancedcreep resistance at high temperatures. The improvement in creepresistance is the result of controlled devitrification of the fusedquartz article. Controlled devitrification is achieved by coating thetubes with a colloidal silica slurry doped with cations of alkalimetals, alkaline earth metals, and rare earth metals, such as, but notlimited to, barium, strontium, and calcium. The metal cations in theslurry promote nucleation and growth of cristobalite crystals (alsoreferred hereinafter as “cristobalite”) into the fused quartz atelevated temperatures. The cristobalite has significantly higherviscosity, and therefore greater creep resistance at elevatedtemperatures, than fused quartz.

Referring to the drawings in general and to FIG. 1 in particular, itwill be understood that the illustrations are for the purpose ofdescribing a preferred embodiment of the invention and are not intendedto limit the invention thereto. FIG. 1 is a cross-sectionalrepresentation of a portion of an article 100 of the present invention.Article 100, which may be, but is not limited to, a crucible, such as acrucible used in silicon wafer growth, or a fused quartz tube, such as aquartz muffle tubes used in the sintering of optical-quality glassboules, comprises a fused quartz body 102. Article 100 further includescoating 104, disposed on an exposed surface of fused quartz body 102.Coating 104 may comprise multiple coatings and may be disposed on morethan one exposed surface of fused quartz body 102.

In one embodiment, article 100, fused quartz body 102, and coating 104are all transparent to visible light (i.e., electromagnetic radiationranging in wavelength from about 4000 angstroms to about 7700angstroms). Transparency is a desirable feature of fused quartz article100. In some applications, such as quartz muffle tubes, transparencypermits viewing of the glass boule within the muffle tube duringprocessing, as well as optical measurement of the boule temperature. Inanother embodiment, article 100 has a melting temperature that is atleast that of cristobalite (about 2270° C.). In yet another embodiment,article 100 is substantially inert to chemical attack by halide gasesand acids. “Substantially inert” is understood to mean that little or noreaction between article 100 and either halide gases or acids occursover time.

Coating 104 comprises a plurality of metal cations, wherein each of thecations has a valence of less than 4, wherein the plurality of metalcations comprises cations of at least one of an alkali metal, analkaline earth metal, a rare earth metal, and combinations thereof. Thepresence of the plurality of metal cations in the surface coating aidsin the nucleation of cristobalite crystals on the surface of fusedquartz body 102, and promotes rapid growth of these crystals into thebulk of fused quartz body 102. In the presence of the plurality ofcations present in coating 104, fused quartz body undergoes a transitionto the cristobalite crystal structure (also referred to hereinafter as“devitrification”) at temperatures in a range from about 1000° C. toabout 1600° C. More preferably, devitrification is carried out at atemperature in a range from about 1350° C. to about 1600° C. In oneembodiment, the plurality of cations comprises at least one of thedivalent alkaline earth cations (also known as Group II cations), suchas the divalent cations of barium (Ba²⁺), calcium (Ca²⁺), strontium(Sr²⁺), and combinations thereof. In another embodiment, the pluralityof cations comprises barium (Ba²⁺) cations. The plurality of cations ispresent within coating 104 in a concentration of at least about 0.1atomic percent. In one embodiment, the concentration of the plurality ofmetal cations within coating 104 is at least 0.5 atomic percent. Inanother embodiment, the concentration of the plurality of metal cationswithin coating 104 is in the range from about 4 atomic percent to about10 atomic percent. Coating 104 has a thickness in a range from about 50nanometers to about 5 microns. In one embodiment, the thickness ofcoating 104 is in a range from about 500 nanometers to about 5 microns.In another embodiment, the thickness is in a range from about 2 micronsto about 5 microns.

Coating 104 is applied to fused quartz body 102 by spraying a slurrycontaining the plurality of metal cations onto the exposed surface offused quartz body 102 to first form a film. The slurry is generallyformed by first dissolving a salt of the metal generate a solutioncontaining a plurality of metal cations. The solution is then combinedwith fumed silica to form the slurry. Silica is added to control thefinal cation concentration at the exposed surface and to facilitatebinding of the cation “dopant” to the surface via subsequent flamepolishing. The slurry is then sprayed onto the exposed surface of fusedquartz body 102. The film is then flame-polished to yield a dense,transparent surface coating 104. Subsequent heat treatment of thearticle 100, now comprising coating 104 and fused quartz body 102,results in devitrification—or conversion to the cristobalite crystalstructure—of the fused quartz body.

At elevated temperatures, fused quartz exhibits significant viscouscreep. Fused quartz articles, such as muffle furnace tubes that are usedin the sintering of fiber optic materials, are frequently maintained athigh temperatures for prolonged periods of time. The creep rate isgreatly diminished by the rapid crystallization, which originates at thelocation of the metal cations in coating 104, of the fused quartz. Thereduction in creep rate extends the lifetime of article 100.

Quartz muffle tubes, for example, are used in the fiber optics industryduring the sintering step in the production of optical-quality glassboules. The tubes are usually suspended vertically, and sintering takesplace at a temperature of about 1500° C. At such temperatures, fusedquartz undergoes viscous creep, which can lead to muffle tube failure.The creep properties of the muffle tube are greatly improved byproducing a controlled devitrified layer of cristobalite on the outerdiameter of the muffle tube. The presence of this crystalline outerlayer extends the projected service time of such tubes to greater thanone year in some cases.

The present invention also includes a method of forming a doped coatingon an exposed surface of a fused quartz article. First, a fused quartzarticle and a slurry of fused silica containing a dopant comprising aplurality of metal cations comprising cations of at least one of analkali metal, an alkaline earth metal, a rare earth metal, andcombinations thereof, with each cation having a positive valence of lessthan 4, are provided. The slurry is prepared by first preparing anaqueous solution of an inorganic compound that comprises the metalcation dopant. For example, a soluble barium compound, such as bariumnitrate, is dissolved in water, the solution is then mixed withcolloidal silica to form the silica slurry. The concentration of themetal cation dopant is present within the silica slurry in aconcentration range from about 13 ppm (parts per million) to about 2000ppm. In one embodiment, the concentration of the metal cation dopant ispresent within the silica slurry in a concentration range from about 800ppm to about 2000 ppm. The silica slurry is then applied to an exposedsurface of the fused quartz article, typically by spraying the slurryonto the exposed surface, and allowed to dry, forming a film on theexposed surface. Alternatively, other application techniques known inthe art, such as, but not limited to, painting and dipping, may be usedto apply the slurry to the exposed surface of the fused quartz article.Additionally, the fused quartz article may be preheated to a temperaturein a range from about 50° C. to about 70° C. The preheating stepincreases the evaporation rate of water from the slurry and helpsproduce a smooth, conformal, doped coating. After drying, the film isflame-polished to yield a dense, transparent surface coating that isdoped with metal cations.

The present invention further includes a method for improving the creepresistance of a fused quartz article. First, a fused quartz article anda slurry of fused silica containing a dopant comprising a plurality ofmetal cations comprising cations of at least one of an alkali metal, analkaline earth metal, a rare earth metal, and combinations thereof, witheach cation having a positive valence of less than 4, are provided. Theslurry is prepared by first preparing an aqueous solution of aninorganic compound that comprises the metal cation dopant. For example,a soluble barium compound, such as barium nitrate, is dissolved inwater, the solution is then mixed with colloidal silica to form thesilica slurry. The concentration of the metal cation dopant is presentwithin the silica slurry in a concentration range from about 13 ppm(parts per million) to about 2000 ppm. In one embodiment, theconcentration of the metal cation dopant is present within the silicaslurry in a concentration range from about 800 ppm to about 2000 ppm.The slurry is then applied to an exposed surface of the fused quartzarticle, typically by spraying the slurry onto the exposed surface andallowed to dry, forming a film on the exposed surface. Alternatively,other application techniques known in the art, such as, but not limitedto, painting the slurry onto the exposed surface and dipping the fusedsilica article into a bath containing the slurry, may be used to applythe slurry to the exposed surface of the fused quartz article.Additionally, the used quartz article maybe preheated to a temperaturein a range from about 50° C. to about 70° C. The preheating stepincreases the evaporation rate of water from the slurry and helpsproduce a smooth, conformal, doped coating. After drying, the film isflame-polished to yield a dense, transparent surface coating. The coatedfused silica article is then heated to a temperature in a range fromabout 1000° C. to about 1600° C., and, preferably, in a range from about1350° C. to about 1600° C., to form cristobalite crystals on the exposedsurface of the fused quartz article. Preferably, devitrification of thecoated fused silica article is carried out at a temperature of about1350° C. The cristobalite crystals enhance the creep resistance of thefused quartz article.

The following example serves to illustrate the features and advantagesof the present invention.

EXAMPLE 1

A fused quartz tube, having an outer diameter of 32 mm and an innerdiameter of 24 mm, was coated with a barium-doped silica coating. Inorder to fuse the barium dopant onto the surface of the fused quartz,barium nitrate first was dissolved in water and mixed with colloidalsilica to form a suspension. The amount of barium nitrate in thesuspension was sufficient to provide about 4.5 atomic % bariumsubstituted on the silicon sites within the coating following flamepolishing. The silica was added to control the final bariumconcentration at the surface and also facilitate binding of the dopantto the surface via subsequent flame polishing. The colloidal silica wasmixed into de-ionized water and adjusted to a pH of 1.0 by adding nitricacid to the solution. The resultant suspension contained 2.0 volume %colloidal silica. The coatings were applied to the outer surface of thetube using a pneumatic spray gun. The as-sprayed coating thickness wasabout 10 microns prior to flame polishing. The barium-doped colloidalsilica coating was fused to the surface using a hydrogen burner attachedto a glass-working lathe. The tube rotation rate was about 5 rpm, withthe burner translating across the tube at approximately 12inches/minute. A total of 3 coats were applied to the tube. Finalcoating thickness was in the range from about 5 microns to about 10microns. Following coating, the tube was annealed at 1100° C. for 30minutes.

While typical embodiments have been set forth for the purpose ofillustration, the foregoing description should not be deemed to be alimitation on the scope of the invention. Accordingly, variousmodifications, adaptations, and alternatives may occur to one skilled inthe art without departing from the spirit and scope of the presentinvention.

1. A fused quartz article, said fused quartz article comprising: a) abody, said body comprising fused quartz; and b) a coating disposed on anexposed surface of said body, said coating comprising a plurality ofmetal cations, each having a valence of less than 4, wherein saidplurality of metal cations comprises cations of at least one of analkali metal, an alkaline earth metal, a rare earth metal, andcombinations thereof, wherein said plurality of metal cations is presentwithin said coating in a concentration of at least about 0.1 atomicpercent, and wherein fused quartz within said body undergoes atransition to a cristobalite crystal structure at a temperature in arange from about 1000° C. to about 1600° C.
 2. The fused quartz articleaccording to claim 1, wherein said fused quartz article is transparentto visible light.
 3. The fused quartz article according to claim 1,wherein said fused quartz article is one of a furnace tube and acrucible.
 4. The fused quartz article according to claim 1, wherein saidfused quartz article is substantially chemically inert with respect tohalide gases and acids.
 5. The fused quartz article according to claim1, wherein said fused quartz article has a melting temperature of atleast that of cristobalite.
 6. The fused quartz article according toclaim 1, wherein said coating has a thickness from about 50 nm to about5 microns.
 7. The fused quartz article according to claim 6, whereinsaid coating has a thickness from about 500 nm to about 5 microns. 8.The fused quartz article according to claim 7, wherein said coating hasa thickness from about 2 microns to about 5 microns.
 9. The fused quartzarticle according to claim 1, wherein said plurality of cationscomprises cations of at least one of barium, calcium, strontium, andcombinations thereof.
 10. The fused quartz article according to claim 1,wherein said at least one metal cation is present within said coating ina concentration of at least about 0.5 atomic percent.
 11. The fusedquartz article according to claim 10, wherein said at least one metalcation is present within said coating in a concentration from about 4atomic percent to about 10 atomic percent.
 12. An outer coating for afused quartz article, said outer coating comprising a plurality of metalcations, wherein said plurality of metal cations comprises cations of atleast one of barium, calcium, strontium, and combinations thereof, andwherein said plurality of metal cations is present within said coatingin a concentration of at least about 0.1 atomic percent, and whereinsaid plurality of cations catalyzes a transition of fused quartz withinsaid fused quartz article to a cristobalite crystal structure at atemperature in a range from about 1000° C. to about 1600° C.
 13. Theouter coating according to claim 12, wherein said outer coating istransparent to visible light.
 14. The outer coating according to claim12, wherein said outer coating has a thickness from 50 nm to about 5microns.
 15. The outer coating according to claim 14, wherein said outercoating has a thickness from 500 nm to about 5 microns.
 16. The outercoating according to claim 15, wherein said outer coating has athickness from about 2 microns to about 5 microns.
 17. The outer coatingaccording to claim 12, wherein said at least one metal cation is presentwithin said coating in a concentration of at least about 0.5 atomicpercent.
 18. The outer coating according to claim 17, wherein said leastplurality of metal cations is present within said outer coating in aconcentration of from about 4 atomic percent to about 10 atomic percent.19. A fused quartz article, said fused quartz article comprising: a) abody, said body comprising fused quartz; and b) an outer coatingdisposed on an exposed surface of said body, said outer coatingcomprising a plurality of metal cations, wherein said plurality of metalcations comprises cations of at least one of barium, calcium, strontium,and combinations thereof, wherein said plurality of metal cations ispresent within said coating in a concentration of at least about 0.1atomic percent, wherein said plurality of cations catalyzes a transitionof fused quartz within said body to a cristobalite crystal structure ata temperature a temperature in a range from about 1000° C. to about1600° C., and wherein said fused quartz article is transparent tovisible light.
 20. The fused quartz article according to claim 19,wherein said fused quartz article is one of a furnace tube and acrucible.
 21. The fused quartz article according to claim 19, whereinsaid fused quartz article is substantially chemically inert with respectto halide gases and acids.
 22. The fused quartz article according toclaim 19, wherein said fused quartz article has a melting temperature ofat least that of cristobalite.
 23. The fused quartz article according toclaim 19, wherein said outer coating has a thickness from about 50 nm toabout 5 microns.
 24. The fused quartz article according to claim 23,wherein said outer coating has a thickness from about 500 nm to about 5microns.
 25. The fused quartz article according to claim 24, whereinsaid outer coating has a thickness from about 2 microns to about 5microns.
 26. The fused quartz article according to claim 19, whereinsaid at least one metal cation is present within said coating in aconcentration of at least about 0.5 atomic percent.
 27. The fused quartzarticle according to claim 26, wherein said wherein said plurality ofmetal cations is present within said outer coating in a concentrationfrom about 4 atomic percent to about 10 atomic percent.
 28. A method offorming a doped coating on an exposed surface of a fused quartz article,the fused quartz article comprising a body comprising fused quartz and acoating disposed on an exposed surface of the body, the doped coatingcomprising a plurality of metal cations, each having a valence of lessthan 4, wherein the plurality of metal cations comprises cations of atleast one of an alkali metal, an alkaline earth metal, a rare earthmetal, and combinations thereof, wherein the plurality of metal cationsis present within the doped coating in a concentration of at least about0.1 atomic percent, the method comprising the steps of: a) providing asilica slurry, the silica slurry being doped with a plurality of metalcations comprising cations of at least one of an alkali metal, analkaline earth metal, a rare earth metal, and combinations thereof; b)providing a fused quartz article; c) applying the silica slurry to anexposed surface of the fused quartz article; d) drying the silica slurryon the exposed surface; and e) fire polishing the exposed surface toform the doped coating on the exposed surface.
 29. The method accordingto claim 28, wherein the step of providing a silica slurry comprisesproviding a silica slurry doped with a plurality of metal cations,wherein the metal cations are selected from the group consisting ofbarium ions, calcium ions, and strontium ions.
 30. The method accordingto claim 29, wherein the step of providing a silica slurry doped with aplurality of metal cations comprises providing a silica slurry dopedwith a plurality of barium ions.
 31. The method according to claim 29,wherein the plurality of metal cations ions is present in the silicaslurry in a concentration range from about 13 ppm to about 2000 ppm. 32.The method according to claim 31, wherein the plurality of metal cationsions is present in the silica slurry in a concentration range from about800 ppm to about 2000 ppm.
 33. The method according to claim 28, whereinthe step of applying the silica slurry to an exposed surface of thefused quartz article comprises spraying the silica slurry onto anexposed surface of the fused quartz article.
 34. The method according toclaim 28, wherein the step of applying the silica slurry to an exposedsurface of the fused quartz article comprises painting the silica slurryonto the exposed surface of the fused quartz article.
 35. The methodaccording to claim 28, wherein the step of applying the silica slurry toan exposed surface of the fused quartz article comprises dipping thefused silica article into a bath containing the silica slurry.
 36. Themethod according to claim 28, further including the step of preheatingthe fused quartz article to a temperature in the range from about 50° C.to about 70° C. prior to applying the silica slurry to the exposedsurface of the fused quartz article.
 37. The method according to claim28, wherein the doped coating has a thickness from about 50 nm to about5 microns.
 38. The method according to claim 37, wherein the dopedcoating has a thickness from about 500 nm to about 5 microns.
 39. Themethod according to according to claim 38, wherein the doped coating hasa thickness from about 2 microns to about 5 microns.
 40. The methodaccording to claim 28, wherein the plurality of metal cations is presentwithin the doped coating in a concentration of at least about 0.5 atomicpercent.
 41. The method according to claim 40, wherein the plurality ofmetal cations is present within the doped coating in a concentrationfrom about 4 atomic percent to about 10 atomic percent.
 42. A method ofimproving the creep-resistance of a fused quartz article, the fusedquartz article comprising a body comprising fused quartz and a coatingdisposed on an exposed surface of the body, the coating comprising aplurality of metal cations, each having a valence of less than 4,wherein the plurality of metal cations comprises cations of at least oneof an alkali metal, an alkaline earth metal, a rare earth metal, andcombinations thereof, wherein the plurality of at least one metal cationis present within the coating in a concentration of at least about 0.1atomic percent, and wherein the body undergoes a transition to acristobalite crystal structure at a temperature in a range from about1000° C. to about 1600° C., the method comprising the steps of: a)providing a silica slurry, the silica slurry being doped with aplurality of metal cations comprising cations of at least one of analkali metal, an alkaline earth metal, a rare earth metal, andcombinations thereof; b) providing a fused quartz article; c) applyingthe silica slurry to an exposed surface of the fused quartz article; d)drying the silica slurry on the exposed surface; e) fire polishing theexposed surface, wherein the silica slurry, after drying forms a dopedcoating on the exposed surface; and f) heating the fused quartz articleand the doped coating to a temperature in a range from about 1000° C. toabout 1600° C., thereby nucleating cristobalite crystals on the exposedsurface, wherein the cristobalite crystals enhance the creep resistanceof the fused quartz article.
 43. The method according to claim 42,wherein the step of providing a silica slurry comprises providing asilica slurry doped with a plurality of metal cations, wherein the metalcations are selected from the group consisting of barium ions, calciumions, and strontium ions.
 44. The method according to claim 43, whereinthe step of providing a silica slurry doped with a plurality of metalcations comprises providing a silica slurry doped with a plurality ofbarium ions.
 45. The method according to claim 42, wherein the pluralityof metal cations is present in the silica slurry in a concentrationrange from about 13 ppm to about 2000 ppm.
 46. The method according toclaim 45, wherein the plurality of metal cations is present in thesilica slurry in a concentration range from about 800 ppm to about 2000ppm.
 47. The method according to claim 42, wherein the step of applyingthe silica slurry to an exposed surface of the fused quartz articlecomprises spraying the silica slurry onto an exposed surface of thefused quartz article.
 48. The method according to claim 42, wherein thestep of applying the silica slurry to an exposed surface of the fusedquartz article comprises painting the silica slurry onto the exposedsurface of the fused quartz article.
 49. The method according to claim42, wherein the step of applying the silica slurry to an exposed surfaceof the fused quartz article comprises dipping the fused silica articleinto a bath containing the silica slurry.
 50. The method according toclaim 42, further including the step of preheating the fused quartzarticle to a temperature in the range from about 50° C. to about 70° C.prior to applying the silica slurry to the exposed surface of the fusedquartz article.
 51. The method according to claim 42, wherein the stepof heating the fused quartz article and the doped coating to atemperature in a range from about 1000° C. to about 1600° C. comprisesheating the fused quartz article and the doped coating to about 1350° C.52. The method according to claim 42, wherein the doped coating has athickness from about 50 nm to about 5 microns.
 53. The method accordingto claim 52, wherein the doped coating has a thickness from about 500 nmto about 5 microns.
 54. The method according to according to claim 53,wherein the doped coating has a thickness from about 2 microns to about5 microns.
 55. The method according to claim 42, wherein the pluralityof metal cations is present within the doped coating in a concentrationof at least about 0.5 atomic percent.
 56. The method according to claim55, wherein the plurality of metal cations is present within the dopedcoating in a concentration from about 4 atomic percent to about 10atomic percent.